A variable valve timing device is a known device applied to an internal combustion engine mounted on a vehicle or the like. The variable valve timing device changes the relative rotation phase of a camshaft with respect to a crankshaft, which is an engine output shaft, to vary the valve timing of engine valves (intake/exhaust valves). As such a variable valve timing device, an oil pressure type variable valve timing device operated by oil pressure such as that described in patent document 1 is known.
The structure of the variable valve timing device described in document 1 will now be described with reference to FIG. 3.
As shown in the drawings, a vane rotor 2, which includes a plurality of (three in the drawing) vanes 3 projecting outward in the radial direction, is fixed to a camshaft 1 so as to be rotatable integrally with the camshaft 1. A generally annular housing 4 is arranged outside the vane rotor 2 so as to be rotatable relative to the vane rotor 2. A cam sprocket 5, which is driven by and coupled to a crankshaft of an internal combustion engine with a chain, is fixed to the housing 4 to be rotatable integrally with the housing 4. Recesses 6, the number of which is the same as the vanes 3, are arranged inside the housing 4. One of the vanes 3 is arranged in each recess 6. The vane 3 accommodated in each recess 6 of the housing 4 defines two oil pressure chambers, namely a retardation chamber 7 and an advancement chamber 8. The retardation chamber 7 is located at the front side of the vane 3 in the rotation direction of the camshaft, and the advancement chamber 8 is located at the rear side of the vane 3 in the rotation direction of the camshaft.
The variable valve timing device includes a lock mechanism that locks the vane rotor 2 and the housing 4 so that the vane rotor 2 and the housing 4 rotate integrally. The lock mechanism includes a lock pin 10, which is movably arranged in a pin hole 9 formed in one of the vanes 3 of the vane rotor 2, and a lock hole 11, which is formed in the cam sprocket 5 and in which the lock pin 10 can be fit. A spring 12 urges the lock pin 10 in a direction in which the lock pin 10 is fitted to the lock hole 11. In a relative rotation range of the vane rotor 2 with respect to the housing 4, the lock pin 10 is located at a position in which it can be fitted to the lock hole 11 when the vane rotor 2 is relatively rotated to a farthest position in a direction opposite to the camshaft rotation direction (hereafter, referred to as most retarded position).
The variable valve timing device includes an oil pressure circuit that adjusts oil pressure to operate the variable valve timing device. In the oil pressure circuit, an oil pump 14, which draws oil from an oil pan 13 and discharges the oil, is connected by an supply oil passage 15 to an oil control valve (hereafter, referred to as the OCV 16). The OCV 16, which is formed as an electromagnetic drive valve controlled by an electronic control unit (hereafter, referred to as the ECU 20) that controls the engine, is connected to a drain oil passage 17, which returns oil to the oil pan 13, a retardation oil passage 18, which is connected to each retardation oil chamber 7, and an advancement oil passage 19, which is connected to each advancement oil chamber 8, in addition to the supply oil passage 15. The OCV 16 switches the one of the supply oil passage 15 and drain oil passage 17 that is connected to each of the retardation oil passage 18 and advancement oil passage 19 so that oil is supplied to or discharged from the retardation oil chambers 7 and the advancement oil chambers 8.
The oil pressure supplied to the retardation oil chambers 7 and the advancement oil chambers 8 acts on the lock pin 10. The oil pressure acts to remove the lock pin 10 from the lock hole 11 against the urging force of the spring 12.
The operation of the variable valve timing device will now be described.
The ECU 20, which serves as a control unit, instructs the OCV 16 to connect the supply oil passage 15 and the advancement oil passage 19 and to connect the drain oil passage 17 and the retardation oil passage 18. This increases the oil pressure in the advancement oil chambers 8 and decreases the oil pressure in the retardation oil chambers 7. The oil pressure chamber difference between the two oil chambers applies force to the vanes 3 that is directed in the rotation direction of the camshaft (hereafter, referred to as the advancement direction). This relatively rotates the vane rotor 2 with respect to the housing 4 in the advancement direction. As a result, the rotation phase of the camshaft 1, which is fixed to the vane rotor 2 in an integrally rotatable manner, is advanced from the rotation phase of the cam sprocket 5. This advances the valve timing of the engine valves, which are opened and closed by the camshaft 1.
When the ECU 20 instructs the OCV 16 to connect the supply oil passage 15 and the retardation oil passage 18 and to connect the drain oil passage 17 and the advancement oil passage 19, the oil pressure in the retardation oil chambers 7 increases and the oil pressure in the advancement oil chambers 8 decreases. The oil pressure chamber difference between the two oil chambers applies force to the vanes 3 that is directed in the direction opposite to the camshaft rotation direction (hereafter, referred to as the retardation direction). This relatively rotates the vane rotor 2 with respect to the housing 4 in the retardation direction. As a result, the rotation phase of the camshaft 1, which is fixed to the vane rotor 2 in an integrally rotatable manner, is retarded from the rotation phase of the cam sprocket 5. This retards the valve timing of the engine valves, which are opened and closed by the camshaft 1.
When the ECU 20 instructs the OCV 16 to stop the supply and discharge of oil for both of the retardation oil passage 18 and the advancement oil passage 19, the vane rotor 2 stops at a position at where the oil pressures in the retardation oil chambers 7 and the advancement oil chambers 8 is balanced. Thus, the present valve timing of the engine valves is kept constant.
When the engine is started, the vane rotor 2 is located at the most retarded position. Due to the lock pin 10 fitted to the lock hole 11, the vane rotor 2 is locked to rotate integrally with the housing 4 at the most retarded position.
When the discharge pressure of the oil pump 14 is sufficiently increased after the engine is started, the ECU 20 instructs the OCV 16 to connect the supply oil passage 15 and the advancement oil passage 19 to supply oil pressure to the advancement oil chambers 8. The oil pressure supplied to the advancement oil chambers 8 also acts on the lock pin 10, and the oil pressure removes the lock pin 10 from the lock hole 11. This releases the locking of the lock mechanism and permits relative rotation of the vane rotor 2 and the housing 4. In this manner, after the starting of the engine, an instruction issued by the ECU 20 when initially supplying oil pressure to the advancement oil chambers 8 instructs the application of a lock releasing oil pressure to release the locking of the lock mechanism.